The attractive properties of lead have been put to good use in the fabrication of all types of buildings from private houses to industrial complexes. Traditionally, when it is necessary to join lead the tried and trusted technique of lead burning is utilised. This joining method cannot be considered ideal because of the skill needed and the environmental and health problems that are associated with the fumes emitted. Furthermore, lead burning is a highly skilled job and there is a general shortage of skilled operators. As a consequence, the initial cost outlay for laying lead sheet as a continuous membrane is unattractive or uneconomic when compared with felt/bitumen alternatives. The initial installation cost outlay should not be the only factor used when comparing the two roofing systems. Felt roofs generally have a life expectancy of only 10-15 years whereas lead roofs last considerably longer. Over an extended period of say 20-100 years a lead roof is likely to be considerably more cost effective. However, the initial high installation costs still tend to discourage architects and builders from specifying lead as a flat roofing continuous membrane material of choice, despite its longevity and attractive physical appearance.
If a simple and cost-effective method of joining lead sheet were available, then this could change the economic equation in favour of lead sheet as a flat roof membrane material.
Possible alternatives to lead burning are friction welding and friction stir welding. The process of friction welding has been known for many years and typically involves causing relative movement between a pair of workpieces while they are urged together so as to generate a plasticised region, stopping the relative movement and allowing the plasticised region to solidify thereby joining the work pieces.
It has also been proposed in the past to join workpieces by use of a non-consumable member which does not form part of the finished joints. An example of this so-called friction stir butt welding technique can be seen in WO 93/10935 (The Welding Institute) and WO 95/26254 (NORSK HYDRO) in which two workpieces are urged together using various shaped tools which cause the region to be joined to plasticise and hence become joined. This technique allows sheet materials to be welded by a solid state process in either butt or lap geometries. The process operates by forcing a rotating tool with a specially shaped probe along the joint line, which causes intense plastic deformation of the immediate surrounding material. The tool is designed to prevent material escaping, and once the tool has passed a given point, a weld is made.
The leading edge of the rotating tool provides frictional heat and subsequent thermal softening in front of the tip, in effect preheating the area about to be bonded. This effect is especially useful in allowing the passage of the tip part of the rotating tool through the material. The greater the area of the shouldered region of the rotating tool in contact with the joint, then the greater the frictional heat available. However, increasing the diameter of the shoulder has practical limitations and tends to produce a side flash.
To date, this process has been successfully demonstrated for many aluminium alloys and also for thermoplastics. However, application of this process to lead sheet results in major difficulties because lead is relatively soft and malleable and the above techniques do not achieve satisfactory results in terms of weld acceptability.
Even if these problems were to be overcome, there is still no known portable machine which could be used on a lead roof to complete the necessary welds. By the very nature of a roof, the welding process must be carried out in situ. The precision required for this type of joint means that a hand-held tool is quite inappropriate for forming a weld of any length. The object of the present invention is therefore to provide a method and apparatus which can be used to form welded joints of the type in question in situ.